The invention relates to a method for operating an internal combustion engine of a motor vehicle in engine braking mode.
In engine braking or overrun mode, in which no additional fuel is supplied to the internal combustion engine and further combustion is thus prevented, any internal combustion engine in a motor vehicle undergoes a certain braking effect due to the charge exchange work to be performed. Compared to conventionally controlled internal combustion engines, which have a predetermined, generally very low braking torque on account of their mechanically controlled valve drive having fixed control times and valve lifts, internal combustion engines having variable-control valve drives (freely adjustable control times), such as variable-control electromagnetic valve drives, for example, generate a variably controllable braking torque of greater magnitude. German Patent Publication DE 195 46 652 A1 describes a method for reducing the speed of a vehicle equipped with a collision avoidance system, in which the vehicle is braked as a function of distance by changing at least one cylinder to compressor drive in the internal combustion engine provided with a variable valve drive.
Furthermore, an internal combustion engine is known from German Patent Publication DE 101 43 330 A1, having electrohydraulic valve control for a motor vehicle which supplies additional hydraulic consumers, such as a supercharger device for compression of the required combustion air, for improving the overall efficiency of the internal combustion engine having excess hydraulic delivery flow to a hydraulic pump for controlling the valves.
The object of the invention is to provide a method for operating an internal combustion engine having a turbocharger device, which achieves a settable increased engine braking effect. In one preferred embodiment of the invention, a spontaneous response characteristic of the internal combustion engine, in particular in overrun mode, is ensured.
According to the invention, the object is achieved by the controlled compression (control of the exhaust valves) of an adjustable air mass (control of the intake valves) in the combustion chamber of at least one cylinder that is switched off (i.e., unfired, or not supplied with fuel in at least one cylinder combustion chamber), making it possible to generate an increased settable braking torque for braking the vehicle.
Furthermore, a spontaneous response characteristic of the internal combustion engine may be ensured at all times by generating a pressure pulse which is produced by the targeted control of the exhaust valves for the spontaneous release of the previously compressed air mass (as a function of the control of the intake valves) and by the targeted impingement of a turbocharger device by this pressure pulse. As a result of the pressure pulse generated in each work cycle, the turbine for the turbocharger device may be continuously maintained at a significantly increased rotational speed. The sluggish starting response of conventional internal combustion engines having a turbocharger device, the turbines of which must be accelerated starting from a very low rotational speed, is avoided by the subject matter of the invention.
In one preferred refinement of the invention, for generating a pulse of maximum pressure all intake valves for every cylinder included for producing the braking effect (preferably all cylinders) are controlled in such a way that the largest possible air mass is drawn into the cylinder combustion chamber (approximately corresponding to control of the intake valves in full-load operation), and the exhaust valves for all cylinders are controlled in such a way that the compressed air mass is not released until just before top dead center (TDC) of the piston.
The desired braking torque is advantageously set by the type of control of the valves, but alternatively or additionally may be achieved by switching between different engine operating modes (two-cycle operation, four-cycle operation, or the like) and/or by the variable number of cylinders used for the braking mode. Setting of the braking torque by control of the valves may be realized in a first embodiment by variably controlling the intake valves in full-load operation (intake of the greatest possible air mass), and variably controlling the exhaust valves for adjusting the compression rate of the air mass provided by the intake valves. In a second embodiment, the intake valves are variably controlled for setting the braking torque by opening the exhaust valves at the latest possible time before TDC. Lastly, in a third possible embodiment the intake valves as well as the exhaust valves are variably controlled for setting a desired braking torque.
For the setting of the desired braking torque by different operating modes, the unfired cylinders may, for example, be switched back and forth between two-cycle operation and four-cycle operation, and/or the number of unfired cylinders may be varied. Since in four-cycle operation one compression and one expansion occurs between two consecutive braking cycles, and the former do not contribute significantly to the braking effect, an optimum combination of the braking effect to be achieved and the high rotational speed of the turbocharger device is achieved in two-cycle operation.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.